Neuronal Loss in the Nucleus Tractus Solitarius and Associated Disturbances of the Hypoxic Ventilatory Response in Rat Offspring Following in‐utero Morphine Exposure

Abstract

The incidence of opioid use during pregnancy, including prescriptions, has become a growing public health concern and has been linked to neonatal abstinence syndrome (NAS), poor neurodevelopmental outcome and increased risk of sudden infant death syndrome (SIDS). Abnormal cardiorespiratory regulation in infants born following in-utero opioid exposure includes reduced vagal tone, delayed arousal, and altered ventilatory responses to hypoxia and hypercapnia. Impairments in ventilatory defense responses and abnormal development of brainstem cardiorespiratory control regions may contribute to infant vulnerability and dampen responses to exogenous stressors leading to SIDS. Although infants with NAS may be treated with morphine, not all infants require pharmacologic treatment. To mimic the latter group of infants, we used a rat model of in utero morphine exposure to investigate the long-term effects it has on respiratory control and brainstem neuronal development of the offspring. Using a rat model of in utero morphine exposure to characterize long-term (10 days) effects on the offspring including: 1) the acute hypoxic (HVR) and hypercapnic ventilatory response (HCVR) as indices of respiratory control (dys)function; and 2) brainstem neuron counts as an indicator of neural development. Pregnant dams received twice daily (morning and afternoon) subcutaneous injections of various doses of morphine (0.5, 1, and 5mg/kg) or saline (control group) during the final week of pregnancy to mimic 3rd trimester maternal opioid use. Following birth, nursing dams and rat pups were left untreated. On postnatal day 10, whole body plethysmography was used to assess the HVR and HCVR in the pups. Brains were removed and prepared for immunohistochemical assessment of neuron cell counts in key brainstem cardiorespiratory control regions. In-utero morphine exposure at any dose (0.5, 1, 5mg/kg) caused a long-term attenuation of the HVR (Fig. 1A), but not the HCVR when compared to pups from in-utero saline treated rats. At the highest dose (5mg/kg), neuron cell numbers were reduced by approximately 50% compared to saline rats (Fig. 1B-C). These data demonstrate a long-term effect of in-utero morphine exposure on respiratory control, which may in part be explained by disrupted brainstem neural development. These cardiorespiratory anomalies could offer insight into the pathophysiology of the increased risk of SIDS seen in infants born from mothers with opioid use.